holloa vs holler

holloa and holler both are nouns.

Word	Noun	Adjective	Verb	Adverb
holloa	Yes	No	No	No
holler	Yes	No	Yes	No

As nouns, holloa and holler are synonyms defined as:

Other synonyms of holloa include bellow, bellowing, holla, hollering, hollo, roar, roaring, yowl.

holloa (noun)	holler (noun)
a very loud utterance (like the sound of an animal)	a very loud utterance (like the sound of an animal)
	a small valley between mountains